Time division multiple access (“TDMA”) systems have multiple subscriber units transmitting in reserved slots to a base station. As illustrated in FIG. 1, these subscriber units 1001–100n are of varying distances from the base station 102. If a first subscriber unit 1001 located far from the base station 102 transmits a burst in one reserved slot and a second subscriber unit 1002 located near to the base station 102 transmits a burst in the following reserved slot, interference is likely to occur. The end of the burst from the first subscriber unit 1001 located far from the base station 102 may lag and interfere with the beginning of the burst from the second subscriber located near the base station.
Allowing for the worst-case propagation delay any subscriber unit 100 may be from the base station 102, i.e., a shortened inbound burst, can prevent this interference. That way, if a subscriber unit 100 is located far from the base station 102, the end of its inbound burst will end in the allotted time reserved for that slot.
Another way to remedy the problem is to advance the transmission of the subscriber unit 100 located far from the base station 102 in such a manner that it does not lag into the transmission of the subscriber unit 100 located near the base station 102 (i.e., the subscriber unit 100 located far from the base station 102 would transmit sooner relative to the outbound bursts than the subscriber unit 100 located near the base station 102). The advantages of the latter method is that system throughput is increased. The room in the inbound burst that would have been allotted for worst-case propagation delay can now be assigned to signaling.
Current time advance methods require that the base station estimate the delay of the inbound signal, transmit an absolute adjustment value to the subscriber unit 100 while keeping a running history of the adjustment values for that subscriber unit 100. TDMA systems (e.g., iDEN, GSM, GPRS, etc.) rely on this method. A disadvantage to the current time advance methods requires the base station 102 to keep a history for each subscriber unit 100, thus burdening it with memory management and fast retrieval times for real-time operation. Furthermore, because the adjustment values are absolute, when channel errors occur, the recovery time until the adjustment values are accurate again is long. The longer it takes for a subscriber unit 100 to recover its time adjustment, the more likely interference will result. Also, because it takes longer for the subscriber unit 100 to recover its time adjustment, a greater number of adjustment values need to be transmitted by the base station 102 in order for the subscriber unit to recover its time advance, thus negatively impacting system throughput.
Thus, there exists a need for a subscriber unit to recover its time advance/delay as quickly as possible.